Hydrogenation versus hydrogenolysis during alkaline electrochemical valorization of 5-hydroxymethylfurfural over oxide-derived Cu-bimetallics

The electrochemical conversion of 5-Hydroxymethylfurfural, especially its reduction, is an attractive green production pathway for carbonaceous e-chemicals. We demonstrate the reduction of 5-Hydroxymethylfurfural to 5-Methylfurfurylalcohol under strongly alkaline reaction environments over oxide-derived Cu bimetallic electrocatalysts. We investigate whether and how the surface catalysis of the MOx phases tune the catalytic selectivity of oxide-derived Cu with respect to the 2-electron hydrogenation to 2.5-Bishydroxymethylfuran and the (2 + 2)-electron hydrogenation/hydrogenolysis to 5-Methylfurfurylalcohol. We provide evidence for a kinetic competition between the evolution of H2 and the 2-electron hydrogenolysis of 2.5-Bishydroxymethylfuran to 5-Methylfurfurylalcohol and discuss its mechanistic implications. Finally, we demonstrate that the ability to conduct 5-Hydroxymethylfurfural reduction to 5-Methylfurfurylalcohol in alkaline conditions over oxide-derived Cu/MOx Cu foam electrodes enable an efficiently operating alkaline exchange membranes electrolyzer, in which the cathodic 5-Hydroxymethylfurfural valorization is coupled to either alkaline oxygen evolution anode or to oxidative 5-Hydroxymethylfurfural valorization.

(1) In the Abstract, the abbreviation of OD-Cu catalyst appears for the first time. It is necessary to explain what it refers to.
(2) In the Catalyst Synthesis section, the authors indicated that "the choice of a two-phase catalyst concept originated in the basic favorable HMFRR reactivity of pure crystalline CuO that we intended to tune by the nm-scale (rather than Angstrom-scale) presence a distinct oxide phase with varying structure and chemisorption characteristic." Literatures should be cited here to support this conclusion.
(3) The authors chose first row transition metals as the M element in MOx, the descriptions on the reason why specific MOx were chosen to perform the reaction especially in alkaline condition, including their respective functions are suggested to add in Introduction part.
(4) For the figure 2 in page 7, since the Cu0 and Cu+ could be distinguished in Cu 2p XPS spectra, Cu LMM is suggested to be supplied.
(5) In line 164-165, the authors claim that "…a very rough Cu surface characterize by many undercoordinated Cu adatoms which likely serve as active sites for activation steps of reactant molecules…", Cu K-edge EXAFS is suggested to be added to acquire relevant local coordination information.
(6) The BET results are quite different over series catalysts, then how the factor about specific surface area on catalytic performance be excluded. In other word, how is the size effect of CuO excluded? Otherwise, the role of surface catalysis in MOx is too speculative without excluding these factors.
(7) For the figure 2 in Page 11, the error bars are lacked. In theory, the experiments should be repeated at least 3 times to be conformable with presented results.
(8) The mechanistic studies are very speculative and lack of support by either experiments or literatures (even if they are for thermocatalytic reaction, some information can be inferred). Particularly the C-OH group of the BAMF could be directly adsorbed into the MOx, this is not considered in this work but likely an important path. The authors focus the competition between the 2e-/2H+ -COH hydrogenation to -CH2OH and its subsequent 2e-/2H+ hydrogenolysis to -CH3, it is better to perform DFT calculation aiming to provide more convincing evidence. In addition, series in-situ experiments such as in situ FTIR should be designed to investigate the adsorption/activation behavior of substrate or intermediated products molecules.
(9) How about the catalytic performance in comparison with peer work especially with other non-noble catalysts. To highlight the performance advantages of OD-Cu/MOx, the evaluation results of single factors such as substrate conversion, product selectivity and other reaction condition especially temperature should be expressed through a radar graph in SI.
(10) In order to further determine the advantages of the two-phase catalyst, it is recommended to assess the performance of the physical mixture of two single oxides for comparison. This necessary contrast is indispensable.
(11) The authors claim that such two-phase structure is induced during the reaction process. The specific structural dynamic evolution needs to be clearer.
(12) How about the stability of OD-Cu/MOx catalysts. Relevant reusability test should be added.
(13) The authors mentioned "Alkaline conditions favor humin formation", but did not give the selectivity of di-and polymerization products. Have the di-and polymerization products been detected and have the carbon balance been calculated in this work?
Reviewer #3 (Remarks to the Author): Recommendation: Minor revisions needed Comments: The topic of this manuscript is relevant for Nature Communications. The manuscript is well written, the characterization methodology is sound, the authors provided enough detail to reproduce their work, and the results are very important for the electrochemical community. However, it contains a few experimental and analytical issues that the authors need to address before it can be accepted for publications. Therefore, I recommend reconsidering after doing minor revisions.
In summary, the authors explored the electrochemical reduction of 5-Hydroxymethylfurfural (HMF) to 5-Methylfurfurylalcohol (MFA) under strongly alkaline reaction environments over oxide-derived Cu bimetallic electrocatalysts. The authors investigated the relationship between the surface catalysis CuO on NiO, Fe2O4, Co3O4 surfaces and the selectivity towards different reduction products such as 2,5-Bishydroxymethylfuran (BHMF) and hydrogen evolution reaction (HER). The authors provide evidence for a kinetic competition between the HER and hydrogenolysis of BHMF to MFA and showed that the product selectivity depends on the electrode composition and current density. However, each material had different (electrochemical) surface area ( Figure 2K) and required different potentials to generate the same current density ( Figure 3) when tested in Rotating Disk Electrodes (RDE) and batch cells; hence, the comparison should not be done as a function of current density but as a function of potential.
Additionally, all the electrodes showed similar electrochemical performance (but different product selectivity) when tested in the in the alkaline exchange membranes (AEM) flow cell. The authors did not measure or report the half-cell potential in the flow cell so this discrepancy in electrochemical performance between the flow and batch systems cannot be explained. The authors are encouraged the compare the performances at equal IR-compensated potentials between the batch and flow system to better understand the results.
For this reason, this manuscript cannot be accepted as it is, and the authors are encouraged to address the following points: 5. The authors report the electrochemical performances (cell potentials and H2 rates) of the MEA-flow cell for the different systems in Figure S16; that is a) CF, b) CuO/CF, c) CuO/NiO/CF, d) CuO/Fe3O3/CF, and e) CuO/CO3O4/CF. The authors show that CF at 10 mA/cm2 nearly identical activity as the other materials. Similar operation voltages were observed at 20 mA/cm2 for all the systems as well. At 30 mA/cm2, all the systems appeared to have better performance (more stable cell voltages) than the CF itself. Hence, it is unclear what the CuO/MxOy is doing at 10 and 20 mA/cm2, and the authors are not encouraged to draw conclusions from these experiments.
6. The fact that the RDE and batch experiments (Fig 3) systems gave such different performances for the different electrodes evaluated while the experiments in flow cell had similar performances suggest that the flow system is not kinetically limited, and the performance might be controlled by different effects such as IR or mass transfer. The authors should measure the half-cell potential in the flow cell to better correlate the performance of the flow system with that of the RDE and batch systems. 7. In Figure 4, the authors are comparing the electrochemical performances at different current densities in the flow MEA cell but Figure S16 show that all the electrodes operated at nearly identical conditions (full cell voltage and H2 rate with and without HMF). Without knowing the half-cell potential, it is difficult to rationalize whether the changes in product selectivity (Fig 4) are caused by differences in (i) potential as previously reported (e.g., ACS Catal. 2019, 9, 9964-9972 andACS Sustainable Chem. Eng. 2020, 8, 4407-4418) or (ii) electrode composition.
8. It is unclear how the authors can conclude that "CuO/Fe2O3/CF showed potential for photoelectrochemical applications with maximum currents of up to 10 mA" as the authors never explored photo-electrochemical activities in this work nor mentioned it anywhere else in the manuscript. I think this line should be removed from the conclusion section. 9. Figure S1 appear to contain the same information as Figure 1. I think Figure S1 should be deleted.
• Line 235: should "cm-2" be "cm2" • Line 355 should say Figure 5 instead of 3. It is fine in the word doc but not in the pdf doc.

Rebuttal letter -Responses to Reviewers' comments
First and foremost, we would like to thank all reviewers for their constructive comments and intensive reviews.
Reviewer 1: Biomass conversion represents a possible means to reduce dependence on these fossil fuels in favor of more environmentally benign and renewable syntheses of fuels and industrially useful compounds. Controlling hydrogenation and hydrogenolysis of HMF is critical to increasing the yield and selectivity of the desired product. This manuscript described the reduction of HMF on oxide-derived Cu bimetallic under alkaline conditions. They verified that the valorization of HMF to MFA via a four-electron process is possible. This result is very important for the fundamental understanding of reaction mechanism, as well as the advanced electrocatalysts design and construct. This manuscript can be accepted after major revision. The detailed comments are listed below: 1. The mass loading is essential for electrochemical performance. In order to give better comparison, the electrochemical activity in mass loading form should also present.
We agree with the reviewer that the mass loading is an essential tool in electrochemistry. As all loadings were 1 mg cm -2 for each prepared electrode, we divide by 1, which is why the values and trends don't change, just their unit.
To satisfy the reviewer's request we have now generated and inserted a mass loading corrected plot ( Figure S16b) for the loading study ( Figure S16a). Figure S16a shows the uncorrected data and Figure S16b the corrected data. It is clear that after correction all overpotentials converge. It is also clear that the loading of 1 mg cm -2 shows the best activity, which confirms our previous approach. Figure S16: Undivided three-electrode cell (UTEC) loading study of CuO/NiO/CF. a) Showing the overpotential at 10 mA cm -2 for different catalyst loadings without (black) and with (green) 10 mM HMF. b) Showing the overpotential at 10 mA cm -2 (mass corrected) for different catalyst loadings without (black) and with (green) 10 mM HMF. The loading of 0 mg cm -2 is out of the plotted range.
2. The ascription of XPS peak should be marked in Figure 2h. The Ni 2p3/2 consists two characteristic peaks, while only one peak in Ni 2p1/2. The detailed explanation should provide.
We thank the reviewer for this comment. It is true that only the Ni 2p3/2 peak consists of two characteristic signals. The publication of Biesinger et al. for example, indicates that our Ni 2p3/2 signal cannot only come from NiO, but must also have formed weak fractions of γ-NiOOH on the CuO/NiO powder surface. 1 We assume that γ-NiOOH was formed during the synthesis in the strongly alkaline milieu and that traces of it were not further oxidized to NiO. Syntheses leading to γ-NiOOH support this hypothesis. 2,3 However, both the XPS data (Figure 2h) and the XRD data (Figure 2b) prove a dominant NiO structure.
We have rewritten and expanded the text to address this circumstance more clearly (Main manuscript Page 5).
3. The electrochemical activity of different electrocatalysts in BET surface area should provide.
The BET corrected activities are given in Figure 3d-f for both MOx and CuO/MOx.

4.
More evidence for mechanism discussion should provide, rather than a proposed reaction pathway. For example, how the MOx phase tune the catalytic selectivity?
To improve our understanding of the mechanistic roles of the individual catalyst components CuO, MOx and CF on the activity, further measurements were added and compared with the existing ones ( Figure S17). The HER and HMFRR polarization curves of CuO/MOx, CuO/CF, MOx/CF and CuO/MOx/CF were plotted for all MOx. In particular, comparatively low HER and HMFRR activity can be seen for NiO/CF and Fe2O3/CF ( Figure S17 a and b). For Co3O4/CF this is also visible, but not quite as strong ( Figure S17c). Furthermore, all CuO/MOx/CF as well as the bimetallic powders CuO/MOx are more active than pure CuO on CF. These two observations suggest a synergistic effect of the combined metal/ metal oxides. 5. Pt mesh counter electrode would give significant contribution for electrochemical activity of cathode, which will gradually deposit on cathode during electrochemical testing. The influence of counter electrode should be considered.
We thank the reviewer for the important objection. Platinum would indeed have a very positive effect on the HER activity and thus a very negative effect on our HMFRR activity and should therefore not participate in the reaction at the cathode.
However, the use of a platinum counter electrode is suitable for our RDE and UTEC systems or preferred over others for the following reasons.
1. The Pourbaix diagram shows that at pH=13 and a potential between 0 to -0.8 V, Pt is stable. 8 2. The platinum counter electrode is only used for the activity tests in the RDE and UTEC. Thus, the reaction time is quite short and the leaching of platinum if there is some would be very low as a result. In addition, the electrolyte is replaced after each measurement. 3. When using a carbon-based electrode, it is very likely that this will reduce our product selectivity in favor of BHH. 9 Despite all this, we have performed measurements for CF with a Pt and a C counter electrode ( Figure  R 1). These show that the Pt counter electrode does not improve the catalytic activity compared to a C counter electrode. Rather, the use of a C counter electrode shows the expected positive effect on the HMFRR (green solid line).

Figure R 1: UTEC measurements comparing CF with a Pt and a C counter electrode.
Undivided three-electrode cell (UTEC) measurements of CF with Pt counter electrode (orange) and C counter electrode (green). UTEC LSV measurements were taken between 0 VRHE to -0,8 VRHE at a scan rate of 10mV s -1 in 0.1 M KOH with (solid line) and without (dashed line) 10 mM HMF without rotation and an electrode area of 1 cm 2 . All measurements are 100% manual internal resistance (IR) corrected.
6. The manuscript should be carefully checked to make it clear to the readers. For example, ' Figure 1k' should be ' Figure 2k'; The details information for reference are incomplete.
We thank the reviewer for this comment and have reviewed the entire manuscript and corrected existing typographical errors.
Reviewer 2: In this work, the authors report an oxide-derived Cu-based two-phase catalysts oxide-derived Cu/MOx and employ it to electro-catalyze 5-hydroxymethylfurfural (5-HMF) valorization. The focus of this work is to demonstrate the nature of the second crystalline metal oxide phase (MOx) on its HER activity and the HMF selectivity over CuO based catalyst. However, their interpretation is speculative and does not provide a clear understanding of the reasons for the various selectivity. For these reasons and details below, the present manuscript does not meet in my view the publication criteria of Nature Communication. Please find below some major points of concern: (1) In the Abstract, the abbreviation of OD-Cu catalyst appears for the first time. It is necessary to explain what it refers to.
We thank the reviewer for the important comment and have defined OD-Cu in the abstract at the beginning (2) In the Catalyst Synthesis section, the authors indicated that "the choice of a two-phase catalyst concept originated in the basic favorable HMFRR reactivity of pure crystalline CuO that we intended to tune by the nm-scale (rather than Angstrom-scale) presence a distinct oxide phase with varying structure and chemisorption characteristic." Literatures should be cited here to support this conclusion.
We agree with the reviewer and have added literature on tuning the activity of CuO by adding second metal oxides or changing the structure of the catalyst and thus the properties.
The choice of a two-phase catalyst concept originated from the good HMFRR reactivity of pure crystalline CuO that we intended to tune by the presence of a distinct second oxide phase at nm scale proximity (rather than by forming a new mixed oxide phase) with varying structure and chemisorption characteristics. 10-13 (Main manuscript, Page 5) Of course, a reference that directly addresses such activity tuning of CuO/MOx in alkaline electrolyte for the HMFRR cannot be given, since we first investigated this. That's why at the beginning of our study we just "intended".
(3) The authors chose first row transition metals as the M element in MOx, the descriptions on the reason why specific MOx were chosen to perform the reaction especially in alkaline condition, including their respective functions are suggested to add in Introduction part.
We thank the reviewer for the helpful comment. And have added to the already existing reasons another central reason to the introduction. (4) For the figure 2 in page 7, since the Cu 0 and Cu + could be distinguished in Cu 2p XPS spectra, Cu LMM is suggested to be supplied.
We thank the reviewer for the reasonable advice and added Cu LMM to Figure S2. (5) In line 164-165, the authors claim that "…a very rough Cu surface characterize by many undercoordinated Cu adatoms which likely serve as active sites for activation steps of reactant molecules…", Cu K-edge EXAFS is suggested to be added to acquire relevant local coordination information.
To support our statement and to follow the valuable suggestion, we have included citations of own and others' XANES and EXAFS studies for CuO under reductive conditions as references in the main manuscript. These works have demonstrated the sustained low averaged Cu coordination number in an oxide-derived state, evidencing the undercoordinated nature of the resulting Cu surface.
They offer rough Cu surfaces, characterized by low average geometric coordination numbers, suggesting a large fraction of undercoordinated Cu adatoms which likely serve as active sites for activation steps of reactant molecules. 12,14,15 (Main manuscript, Page 6) These clearly show that one arrives at lower coordination numbers (CN<12), suggesting enhanced roughness.
(6) The BET results are quite different over series catalysts, then how the factor about specific surface area on catalytic performance be excluded. In other word, how is the size effect of CuO excluded? Otherwise, the role of surface catalysis in MOx is too speculative without excluding these factors.
We thank the reviewer for this important comment and agree that the size effect should be investigated. Unfortunately, it was not possible to make clear statements about the particle sizes by our TEM data ( Figure S6), because the particles agglomerated. For the same reason, it was also not possible to determine the particle size distribution.
Since we are aware of the impact the size effect might have, we tried to get more detailed results with HR-STEM analysis ( Figure S7). However, also HR-STEM only supports our previous statement that the particle size can be classified between 5-100 nm. Comparing this size with the manufacturer's specification for the commercial CuO of <50 nm, however, a particle size effect seems negligible. Normally, one would assume that a smaller particle size would result in a higher surface area and thus more accessible active sites. This would then be visible by a higher activity. However, this is not the case, our synthesized CuO shows a higher BET surface area (Figure 2k) and a comparable BET corrected HMFRR activity (Figure 3e). (7) For the figure 2 in Page 11, the error bars are lacked. In theory, the experiments should be repeated at least 3 times to be conformable with presented results.
The indication of errors should indeed be included, we thank the reviewer for this comment. We share the same opinion as the reviewer, why we repeated our experiments at least 4 times. Due to the high reproducibility, small relative errors of 2-4% for the FEproducts and 3-5% for FEH2 resulted. Therefore, and to preserve both clarity and aesthetics, we did not include the error bars in Figure 4. We show this as an example in Figure S18. Nevertheless, it is important for the reader to know how high the deviations are, so we have included them in the caption of Figure 4. In addition, we added Figure S18 to the SI. CuO/NiO/CF as a catalyst. Product color code stays as in a)-e). g)-i) Scatter plot of the product selectivity preference. HMF conversion over MFA/BHMF selectivity ratio, calculated by SMFA/SBHMF for all CuO/MOx catalysts on CF at different current densities for 30 min. The color code stays the same as before. Cell reaction conditions: 0.1 M KOH with 10 mM HMF as catholyte (100 mL), 0.1 M KOH as anolyte (100 ml), 5 cm 2 electrode area, nickel foam (NF) as anode and a flow rate of 25 mL min -1 , at 10-30 mA cm -2 for 30 min. Error bars with relative errors of 2-4% for FEproducts, 3-5% for FEH2 and 1-3% for XHMF were added.
(8) The mechanistic studies are very speculative and lack of support by either experiments or literatures (even if they are for thermocatalytic reaction, some information can be inferred). Particularly the C-OH group of the BAMF could be directly adsorbed into the MOx, this is not considered in this work but likely an important path. The authors focus the competition between the 2e -/2H + -CHO hydrogenation to -CH2OH and its subsequent 2e -/2H + hydrogenolysis to -CH3, it is better to perform DFT calculation aiming to provide more convincing evidence. In addition, series in-situ experiments such as in situ FTIR should be designed to investigate the adsorption/activation behavior of substrate or intermediated products molecules.
We thank the reviewer for this comment. We agree that detailed Quantum chemical calculations of reaction pathways would be highly desirable in this case. Unfortunately, such theoretical computational analyses are outside the scope of our research expertise, and collaborating theoretical groups, upon request, considered this too formidable a task, given the vast configuration space of the organic molecules and the solvent molecules. We are afraid that we have to pass on computational support for our experimental data this time.
We also agree with the reviewer that in-situ FTIR would in principle be a very suitable method to clarify further details of the reaction mechanism and surface intermediates. This is why we now performed measurements for this. However, as we describe in the following, FTIR turned out to be a suboptimal characterization technique under our chosen reaction conditions: The correct choice of the prism was very difficult, because the prism crystal had to be stable at pH=13, the penetration depth had to be high enough for adequate signal intensity and the refractive index had to be higher than that of the electrolyte, HMF and its products. The high pH value alone limits the possibilities already very much to for example diamond, germanium, silicon or CaF2. 16 Germanium, however, also falls out due to the low penetration depth. Even at different mirror angles, the signal intensity remains too weak. Even though a diamond prism has the best properties for our requirements, we could not use it because no supplier produces a diamond prism in the size and shape, we would need for our setup. In the end, only the Si (pH stability marginal) and CaF2 prisms remained. The refractive index of CaF2 varies between 1.39-1.42. Comparatively, this refractive index is below that of for example HMF with 1.56. Thus, actually also the Si and CaF2 prisms are not optimal options. 16 Nevertheless, we have performed measurements with these prisms (Figure R 2).
For this purpose, it was necessary to perform a background measurement with KOH so that the OH band of the HMF and the BHMF would not be masked by the electrolyte signal. For the CaF2 prism it was not possible to obtain adequate measurement results, as shown in Figure R 2a. The Si prism showed characteristic signals that could be assigned to HMF, but only at strongly increased HMF concentration of 1M HMF (Figure R 2b). After 30 minutes of electrolysis at 20 mA cm -2 with the CuO/CF catalyst, however, it should have been visible that at least the signal of the C=O band decreases in favor of the OH band, since the hydrogenation of HMF to BHMF should have been observed here. After 30 minutes reaction time and the high HMF concentration, the formation of by-products such as humins also becomes problematic.
Finally, it was not possible to perform meaningful and reproducible measurements with the FTIR. (9) How about the catalytic performance in comparison with peer work especially with other non-noble catalysts. To highlight the performance advantages of OD-Cu/MOx, the evaluation results of single factors such as substrate conversion, product selectivity and other reaction condition especially temperature should be expressed through a radar graph in SI.
We thank the reviewer for the reasonable comment. We added a radar plot to the SI ( Figure S20). However, we would like to add that a comparison here is difficult for several reasons. Firstly, there is no literature that uses a pH value above 10. Furthermore, the avoidance of noble metal catalysts in comparable pH ranges is also difficult. Due to the different initial concentrations of HMF, the potential at 10 mA cm -2 should also be considered with caution. Last but not least, we have omitted the temperature comparison, because all measurements are made at room temperature, since an increase in temperature can promote side reactions. Nevertheless, we hope to have met the demand of the reviewer to a sufficient extent.

Figure S 20: Comparison with the literature presented in a radar plot.
Performance and reaction parameters like the HMF conversion (XHMF), selectivity towards BHMF (SBHMF) and MFA (SMFA), the potential at 10 mA cm -2 (E10mA/cm2), pH and initial HMF concentration (cHMF) are compared between CuO/Fe2O3/CF and data from the literature. [17][18][19] (10) In order to further determine the advantages of the two-phase catalyst, it is recommended to assess the performance of the physical mixture of two single oxides for comparison. This necessary contrast is indispensable.
We thank the reviewer for this important comment and agree with the reviewer that this is an important addition. We have performed such a study for the CuO/Fe2O3 catalyst in the RDE configuration. It is visible that the activity of the physically mixed catalyst (Figure S14, yellow) is slightly worse than in the best case comparable to our presented co-precipitated catalyst (Figure S14, purple). We added text to the Main manuscript and the plot to the SI.

Page 8 main manuscript:
We can also add that our (co-)precipitation-calcination (air) synthesis protocol has slight catalytic advantages over the physical mixing of the individual precipitated metal oxides ( Figure S14).

Figure S 14: RDE three-electrode measurements of CuO/Fe2O3 compared to physical mixed CuO and Fe2O3
. RDE measurements of the mixed metal oxide CuO/Fe2O3 (purple) and the physical mixed oxide CuO and Fe2O3 (yellow). All RDE LSV measurements were taken between 0 VRHE to -0,6 VRHE at a scan rate of 10 mV s -1 in 0.1 M KOH with (solid line) and without (dashed line) 10 mM HMF at 2500 rpm with an electrode surface area of 0.19 cm 2 and a catalyst loading of 0.04 mg. All measurements are 100% manual internal resistance (IR) corrected.
(11) The authors claim that such two-phase structure is induced during the reaction process. The specific structural dynamic evolution needs to be clearer.
We thank the reviewer for this comment. We agree that an elucidation of this dynamic would be helpful, but at the same time we think that in-situ EXAFS/XAS measurements would be necessary. Unfortunately, we are not able to establish such measurements within the framework of this study. However, we further base our hypothesis on existing literature from us and other groups on comparable systems. 12,14,15 (12) How about the stability of OD-Cu/MOx catalysts. Relevant reusability test should be added.
We thank the reviewer for this comment. We agree with the reviewer that stability tests are important.
That's why we performed a stability test for the most MFA selective catalyst CuO/Fe2O3/CF (Figure  5a). Since the focus of this study is primarily on the proof of concept of alkaline HMF hydrogenolysis, we feel it is appropriate in this context to show the stability of the most relevant catalyst over 5 catalytic cycles.
(13) The authors mentioned "Alkaline conditions favor humin formation", but did not give the selectivity of di-and polymerization products. Have the di-and polymerization products been detected and have the carbon balance been calculated in this work?
We thank the reviewer for this important and relevant comment. The problem in detecting these di-and polymerization products is their structural diversity. 9 Thus, on the one hand, even with NMR, for example, it is difficult to detect and define exact structures. This makes the calibration of any measurement method practically impossible. Many of these molecules are also not commercially available in pure form, even if one can say with certainty exactly which structures are formed. However, we have performed mass spectroscopy measurements after 30 minutes of electrolysis with CuO/CF, which show that di-and polymers are formed (Figure R 3, orange area). The topic of this manuscript is relevant for Nature Communications. The manuscript is well written, the characterization methodology is sound, the authors provided enough detail to reproduce their work, and the results are very important for the electrochemical community. However, it contains a few experimental and analytical issues that the authors need to address before it can be accepted for publications. Therefore, I recommend reconsidering after doing minor revisions.
In summary, the authors explored the electrochemical reduction of 5-Hydroxymethylfurfural (HMF) to 5-Methylfurfurylalcohol (MFA) under strongly alkaline reaction environments over oxide-derived Cu bimetallic electrocatalysts. The authors investigated the relationship between the surface catalysis CuO on NiO, Fe2O4, Co3O4 surfaces and the selectivity towards different reduction products such as 2,5-Bishydroxymethylfuran (BHMF) and hydrogen evolution reaction (HER). The authors provide evidence for a kinetic competition between the HER and hydrogenolysis of BHMF to MFA and showed that the product selectivity depends on the electrode composition and current density. However, each material had different (electrochemical) surface area ( Figure 2K) and required different potentials to generate the same current density (Figure 3) when tested in Rotating Disk Electrodes (RDE) and batch cells; hence, the comparison should not be done as a function of current density but as a function of potential.
Additionally, all the electrodes showed similar electrochemical performance (but different product selectivity) when tested in the in the alkaline exchange membranes (AEM) flow cell. The authors did not measure or report the half-cell potential in the flow cell so this discrepancy in electrochemical performance between the flow and batch systems cannot be explained. The authors are encouraged the compare the performances at equal IR-compensated potentials between the batch and flow system to better understand the results.
For this reason, this manuscript cannot be accepted as it is, and the authors are encouraged to address the following points: 1. The authors show the XPS spectra of CuO, NiO, Fe2O3, and Co3O4. Do the authors observe any change in oxidation state after reaction?
We thank the reviewer for the important comment. Indeed, we have been able to detect a change in the oxidation state of the catalysts after electrolysis. (Figure R 4). Figure R 4 a-e) shows a change in oxidation state for all metal oxides. However, after electrolysis, metallic (M 0 ) and oxidic (M 1-3+ ) mixed states are obtained. 1,7,20 This may be due to the fact that the reduction from oxidic to metallic was not completed during the reaction or that the catalysts are oxidized again after the reaction on air. Thus, it is difficult to make an exact statement by ex-situ XPS. We thank the reviewer for the very helpful and reasonable comment. We agree and added the data to the SI (Figure S17) as well as text to the main manuscript (Main Manuscript, Page 9).
At the same time, however, Figure S17 shows that CF alone does not necessarily lead to a generally increased performance. Here it becomes clear once again that the combined CuO/MOx metal oxides with or without CF support bring an increase in activity. 3. What percentage of IR correction did the authors apply?
We thank the reviewer for the question and the resulting important additions. We have applied 100% manual IR correction. For clarification, we added this information to all captions.
4. The normalization based on BET can be misleading as not all the area is electrochemically active. The authors are recommended to obtain the real electroreactive surface area (ECSA) based on capacitance. If that is not available, the authors are recommended to just use the BET of the active phase, CuO, to evaluate the trends.
We thank the reviewer for the important comment and agree that the exact determination of the ECSA would provide even more information here. Unfortunately, due to the complexity caused by the catalysts in combination with HMF in the electrolyte, we were not able to determine it or produce reliable results. We also believe that the bifunctionality of reducing HMF and forming protons and hydrogen at the same time could lead to falsification of the ECSA. Therefore, we follow the suggestion of the reviewer and have corrected all CuO/MOx polarization curves with the BET surface area of CuO (Figure R 5). However, all potential curves are corrected with the same value, which preserves the trend of Figure 3c and only increases the current density.  Figure 3c). All RDE LSV measurements were taken between 0 VRHE to -0,6 VRHE at a scan rate of 10 mV s -1 in 0.1 M KOH with (solid line) and without (dashed line) 10 mM HMF at 2500 rpm with an electrode surface area of 0.19 cm 2 and a catalyst loading of 0.04 mg. All measurements are 100% manual internal resistance (IR) corrected.
5. The authors report the electrochemical performances (cell potentials and H2 rates) of the MEA-flow cell for the different systems in Figure S16; that is a) CF, b) CuO/CF, c) CuO/NiO/CF, d) CuO/Fe2O3/CF, and e) CuO/Co3O4/CF. The authors show that CF at 10 mA/cm 2 nearly identical activity as the other materials. Similar operation voltages were observed at 20 mA/cm 2 for all the systems as well. At 30 mA/cm 2 , all the systems appeared to have better performance (more stable cell voltages) than the CF itself. Hence, it is unclear what the CuO/MxOy is doing at 10 and 20 mA/cm 2 , and the authors are not encouraged to draw conclusions from these experiments.
We thank the reviewer for the comprehensible comment. Since a lot of data is presented in Figure S19 (formerly Figure S16), we try to explain that the performances are different, even if slightly, already at 10 and 20 mA cm -2 . Basically, an increasing HER rate is accompanied by an increasing cell potential. This is also obvious since especially CF but also CuO require high overpotentials for pure HER without HMF (Figure 3).
We agree with the reviewer that at 10 mA cm -2 all catalysts are in a comparable potential range. However, the performance additionally weighted by XHMF, FEx and Sx is different (Figure 4). This is not contradictory because at 10 mA cm -2 the HER rate is low for all catalysts and thus the larger potential driver (HER) in our opinion is comparatively insignificant and the cell potential of the different catalysts behaves comparably. Furthermore, at this current density it is not so important for the cell potential whether BHMF or MFA is preferentially formed as long as the HMF concentration is still high and HMF is primarily converted.
At 20 mA cm -2 , however, a potential difference becomes more obvious. Here, the cell potentials of the catalysts with higher HER activity (FEH2, Figure 4) such as CuO/CF, CuO/NiO/CF and CuO/Co3O4/CF are already slightly higher than those of CF at the beginning. The cell potential and the FEH2 of CuO/Fe2O3/CF are more comparable to CF at 20 mA cm -2 . However, in the last minutes of the electrolysis, and hence at low HMF concentration, the cell potential for CF increases sharply and is thus significantly higher than for all other catalysts. Due to the decreasing HMF concentration, the H2 production at the CF catalyst now also increases, whereas the other catalysts remain more selective for HMF and therefore also work at lower potentials.
We think that we included these correlations in the discussion part of the main manuscript. However, we wanted to avoid such a detailed analysis of Figure S19 in the main manuscript in order not to digress too far. We hope to comply with the will of the reviewer.
6. The fact that the RDE and batch experiments (Fig 3) systems gave such different performances for the different electrodes evaluated while the experiments in flow cell had similar performances suggest that the flow system is not kinetically limited, and the performance might be controlled by different effects such as IR or mass transfer. The authors should measure the half-cell potential in the flow cell to better correlate the performance of the flow system with that of the RDE and batch systems.
We thank the reviewer for this important comment and agree that the determination of the half-cell potential by inserting a reference electrode would be helpful. Unfortunately, due to the MEA configuration and the specifications of the cell setup, e.g. the thickness of the gasket, it is not possible for us to place a reference electrode in the system in such a way that reliable results are generated. Therefore, we tried to bridge the gap between three electron measurements and the MEA flow cell as good as possible with the UTEC configuration. Although we are aware or agree with the reviewer that, for example, the electrolyte flow and other specifications of the MEA flow cell can influence the halfcell potential, we believe we have found a good approximation with the UTEC configuration.
7. In Figure 4, the authors are comparing the electrochemical performances at different current densities in the flow MEA cell but Figure S16 show that all the electrodes operated at nearly identical conditions (full cell voltage and H2 rate with and without HMF). Without knowing the half-cell potential, it is difficult to rationalize whether the changes in product selectivity (Fig 4) are caused by differences in (i) potential as previously reported (e.g., ACS Catal. 2019, 9, 9964-9972 and ACS Sustainable Chem. Eng. 2020, 8, 4407-4418) or (ii) electrode composition.
We thank the reviewer for the important comment. And agree that the half-cell potential can have an impact on product selectivity. However, in our opinion, the HER is the more important factor here. Looking at the H2 production rates without HMF, they are quite comparable. However, when HMF is added to the reaction, the H2 rates are different. For example, CuO/Fe2O3/CF shows the highest H2 production rate without HMF and the lowest with HMF. At the same time, CuO/Fe2O3/CF shows the lowest cell potential. These observations confirm the data from Figure 3 that HMFRR takes place at lower potentials and also suggest that with CuO/Fe2O3/CF more adsorbed protons are selectively used for HMFRR instead of HER. Assuming that the NF on the anode always makes comparable contributions to the cell potential at the used current densities, this would therefore imply that CuO/Fe2O3/CF has a lower half-cell potential compared to the other catalysts while making HMFRR more selective. This would also be in agreement with the data from Figure 3. As already described in comment 6, we can't introduce a reference electrode into the MEA flow cell system, we nevertheless believe that in this case the argumentation is conclusive even without such data.
8. It is unclear how the authors can conclude that "CuO/Fe2O3/CF showed potential for photoelectrochemical applications with maximum currents of up to 10 mA" as the authors never explored photo-electrochemical activities in this work nor mentioned it anywhere else in the manuscript. I think this line should be removed from the conclusion section.
We thank the reviewer for this advice and removed this line from the text.
9. Figure S1 appear to contain the same information as Figure 1. I think Figure S1 should be deleted.
We thank the reviewer for this advice and removed Figure S1 from the SI.
• Line 235: should "cm-2" be "cm2" • Line 235: Cu foam already defined in line 233 • Line 250 should say Figure 3 instead of Figure 1. It is fine in the word doc but not in the pdf doc • Line 332 should say Figure 4 instead of 2. It is fine in the word doc but not in the pdf doc.
• Line 355 should say Figure 5 instead of 3. It is fine in the word doc but not in the pdf doc.
• Line 386 replace "Alkaline Membrane HMF Electrolyzer" for "AME" We thank the reviewer for carefully reading and corrected the typos. The reviewer concerns on the manuscript has been well estabilished. The manuscript can be accepted in this journal.

Summary: Revisions needed
Comments: The topic of this manuscript is relevant for Nature Communications. The manuscript is well written, the characterization methodology is sound, and the authors provided enough detail to reproduce their work. The authors addressed some of the concerns that the reviewers identified in the first review and provided additional information. However, there still appears to be some experimental flaws that the authors need to address before the manuscript can be accepted for publication. Therefore, I recommend reconsidering after doing the recommended revisions.
In summary, the authors explored the electrochemical reduction of 5-Hydroxymethylfurfural (HMF) to 5-Methylfurfurylalcohol (MFA) under strongly alkaline reaction environments over oxide-derived Cu bimetallic electrocatalysts. The authors investigated the relationship between the surface catalysis of MOx (NiO, Fe2O4, Co3O4) surfaces tune the selectivity of CuO towards different reduction products such as 2,5-Bishydroxymethylfuran (BHMF) and hydrogen evolution reaction (HER). The authors provide evidence for a kinetic competition between the HER and hydrogenolysis of BHMF to MFA and showed that the product selectivity depends on the electrode composition and current density. However, the trend in performance for the CuO/MOx systems changed when testing in RDE and undivided batch systems; hence, it is unclear if the reported change in performance is associated with the secondary metal, CuO/MOx deposition on the carbon felt (CF) for batch testing, or electrochemical reaction conditions.
The CuO/MOx electrodes were also tested in an alkaline exchange membranes (AEM) flow cell and showed different performance to that on the RDE and undivided batch system. The CuO/MOx systems were operated at high HMF conversions (>57% for 10 mA/cm2, >85% for 20 mA/cm2, and >92% for 30 mA/cm2); hence, the discussion of changes in selectivity with current is not appropriate because the main reagent is mostly consumed, and the system is "forced" to do side reactions to produce a constant rate (current). As the RDE and batch experiment show in Figure 3, the CuO/MOx systems operated at different potentials to generate the same cell with up to 0.3 V difference. Hence, the difference in performance as a function of electrode composition shown in Figure 4 for the flow cell can also be also (partially) due to potential effects. For this reason, the authors cannot conclude that the differences are due to the nature of the secondary metal oxide.
For these reasons, this manuscript cannot be accepted as it is, and the authors are encouraged to address the following points: Electrode synthesis and characterization 1. The authors provided the performance of physically mixed oxides in the revised submission ( Figure  S14), which shows that physical mixing CuO2 and Fe2O3 provides similar performance as co-precipitated CuO2/Fe2O3. Does this also happen for the other key CuO/NiO/CF, d) CuO/Fe2O3/CF, and e) CuO/CO3O4? 2. If the performance of the co-precipitated and physically mixed system is similar, the authors need to provide the characterization of the physically mixed system as well.
Electrochemical performance of the RDE and batch system 1. The authors provided the IR-corrected performance of the different CuO/MOx material in the revised manuscript RDE and batch system, Figure 3c and g. The IR and mass transfer effects are completely different in both systems, so it is hard to do a clean comparison of the absolute values and the trends should be compared instead. After close examination, the trends do not always appear to be the same for the two reaction systems. For example, CuO,HMF (deposited in CF) in batch had the worst performance in batch but CuO,HMF had the second-best performance in RDE. Similarly, CuO/NiO (deposited in CF) had the best performance in batch cell (with and without HMF) while CuO/Fe2O3,HMF had the best performance in RDE. Are the changes in trends associated with the deposition of the metal on the CF for batch (and flow) testing or is it experimental error? The authors must address this difference in trend.
2. Additionally the shape of the curves shown in Figure 3c of some CuO/MOx systems (CuO,HMF, CuO/NiO,HMF and CuO/Co3O4,HMF as well as CuO/Fe2O3 to some extend) in the RDE do not follow the traditional exponential trend. Hence, the authors are encouraged to revise the data and perform different IR corrections to match the data between the RDE and batch cell.
1. The authors indicated in the rebuttal letter that they cannot place a reference electrode in the flow cell. Per Figure 3g, there is a 0.3 V difference between the CuO/MOx electrodes to generate the same current, so the potential at which the electrodes operated in the flow-cell to generate the same current is also different. How do the authors know that the small difference in performance observed with different secondary metal at 10 mA/cm2 is caused by the nature of the secondary metal and not by operating at each system at different potentials? The authors must address this critical point. Since the half-cell potential cannot be measured, the authors are recommended to operate the flow system at different currents (i.e., potentials) but similar conversion levels to demonstrate whether the current/potential affect the product selectivity for a given CuO/MOx combination. Table S3 summarizes the performance of the flow cell after 30 min of reaction. All the catalytic systems were operating at ≈90% HMF conversion at 20 mA/cm2; hence, the discussion of changes in selectivity with current is not appropriate because the main reagent is mostly consumed, and the system is "forced" to do side reactions (H2) to produce a constant rate (current). The authors should operate the system at a similar, lower conversion level (≈10%) for all the test to truly observe the effects of current. The authors are encouraged to run the system with 10X higher concentration of HMF, lower catalyst loadings, and/or lower current densities as well.

Figures 4a to 4e and
3. Figure 4c shows the selectivity of CuO/NiO/CF at 10, 20, 30 mA/cm2 after 30 min of reaction and Figure 4f shows the selectivity of the CuO/NiO/CF at 20 mA/cm2 at 0-15, 15-30, 30-45, and 45-60 min. This reviewer would expect the selectivity of CuO/NiO/CF in Figure 4c at 20 mA/cm2 and 30 min to be the same as the selectivity in Figure 4f at 30 min and 20 mA/cm2 but it is not. Figure 4c shows a 20% H2 and 27% MFA selectivity while Figure 4f shows 71% H2 and 0% MFA selectivity. Why are the results so different even when the electrode, current, and reaction time are the same? The authors must address this inconsistency.
4. Figure 4 g-I show that there were small to negligible effect of the nature of the secondary metal support on CuO performance at 20-30 mA/cm2. For example, at 10 mA ( Figure 4g) CuO/NiO/CF and CuO/Fe2O3/CF are at the opposite side of the plot with different SMFA/SBHMF selectivities; however, at 20 mA/cm2 CuO2/NiO/CF and CuO/Fe2O3/CF both have similar SMFA/SBHMF selectivities. Finally, at 30 mA/cm2 both CuO2/NiO/CF and CuO/Fe2O3/CF have the same SMFA/SBHMF selectivities. However, the authors the conclusion that "the nature of the secondary metal oxide will tune the HMF reduction performance of CuO". The authors are advised to operate the system at low conversion to mitigate effect of conversion to properly investigate the role of the nature of the secondary metal on the CuO performance.
5. Figure 4 shows the performance of selected catalytic systems in the MEA-flow cell. Figure 4f shows the conversion and FE as a function of time on stream for a constant current of 20 mA/cm2 and reveals that the conversion increases from 60% at 0-15 min to >90% conversion at 15-30 min of reaction. Did the authors run the cell under continuous recycle of the electrolyte? If so, this reviewer did not catch this from the text.
6. If not running under continuous recycle, it is unclear how the conversion increases in continuous flow reactor running in a single pass configuration. The authors must address this.

Other
• The nomenclature of the catalyst names in Figure 4 is inconsistent with the text and other Figures. For example, CuO/NiO/CF is called CuO NiO CF in Figure 4c.

Rebuttal letter -Responses to Reviewers' comments
First and foremost, we would like to thank all reviewers for their constructive comments and detailed reviews.

Reviewer 1:
The reviewer concerns on the manuscript has been well estabilished. The manuscript can be accepted in this journal.
We thank the reviewer for his positive verdict.

Summary: Revisions needed
Comments: The topic of this manuscript is relevant for Nature Communications. The manuscript is well written, the characterization methodology is sound, and the authors provided enough detail to reproduce their work. The authors addressed some of the concerns that the reviewers identified in the first review and provided additional information. However, there still appears to be some experimental flaws that the authors need to address before the manuscript can be accepted for publication. Therefore, I recommend reconsidering after doing the recommended revisions.
In summary, the authors explored the electrochemical reduction of 5-Hydroxymethylfurfural (HMF) to 5-Methylfurfurylalcohol (MFA) under strongly alkaline reaction environments over oxide-derived Cu bimetallic electrocatalysts. The authors investigated the relationship between the surface catalysis of MOx (NiO, Fe2O4, Co3O4) surfaces tune the selectivity of CuO towards different reduction products such as 2,5-Bishydroxymethylfuran (BHMF) and hydrogen evolution reaction (HER). The authors provide evidence for a kinetic competition between the HER and hydrogenolysis of BHMF to MFA and showed that the product selectivity depends on the electrode composition and current density. However, the trend in performance for the CuO/MOx systems changed when testing in RDE and undivided batch systems; hence, it is unclear if the reported change in performance is associated with the secondary metal, CuO/MOx deposition on the carbon felt (CF) for batch testing, or electrochemical reaction conditions.
The CuO/MOx electrodes were also tested in an alkaline exchange membranes (AEM) flow cell and showed different performance to that on the RDE and undivided batch system. The CuO/MOx systems were operated at high HMF conversions (>57% for 10 mA/cm 2 , >85% for 20 mA/cm 2 , and >92% for 30 mA/cm 2 ); hence, the discussion of changes in selectivity with current is not appropriate because the main reagent is mostly consumed, and the system is "forced" to do side reactions to produce a constant rate (current). As the RDE and batch experiment show in Figure 3, the CuO/MOx systems operated at different potentials to generate the same cell with up to 0.3 V difference. Hence, the difference in performance as a function of electrode composition shown in Figure 4 for the flow cell can also be also (partially) due to potential effects. For this reason, the authors cannot conclude that the differences are due to the nature of the secondary metal oxide.
For these reasons, this manuscript cannot be accepted as it is, and the authors are encouraged to address the following points: Electrode synthesis and characterization 1. The authors provided the performance of physically mixed oxides in the revised submission ( Figure  S14), which shows that physical mixing CuO and Fe2O3 provides similar performance as co-precipitated CuO2/Fe2O3. Does this also happen for the other key CuO/NiO/CF, d) CuO/Fe2O3/CF, and e) CuO/CO3O4? This is a good comment. We have now performed additional experiments to improve our manuscript. We have prepared physically mixed CuO/NiO and CuO/Co3O4 catalysts and tested them at the RDE scale. Our results evidence (as for CuO/Fe2O3) comparable activity for all physically mixed and coprecipitated catalysts. We have presented all these data in the new Supplementary Fig. 14 a-c. 2. If the performance of the co-precipitated and physically mixed system is similar, the authors need to provide the characterization of the physically mixed system as well.

Supplementary Fig. 14: RDE three-electrode measurements and powder XRD characterization of CuO/MOx compared to physically mixed CuO and MOx. a)-c) RDE measurements of the mixed metal oxides
We fully agree with the reviewer that a characterization comparison between the co-precipitated and physically mixed catalysts would further strengthen our results. Therefore, we investigated the crystal structure of the physically mixed catalysts by powder XRD (Supplementary Fig. 14 d-f). The results show the same reflections for the two synthesis routes, reconfirming the formation of two phases instead of a two-metal oxide mix phase, with mainly CuO tenorite visible for all catalysts (Figure 2 and Supplementary Fig. 14 d-f).
Electrochemical performance of the RDE and batch system 1. The authors provided the IR-corrected performance of the different CuO/MOx material in the revised manuscript RDE and batch system, Figure 3c and g. The IR and mass transfer effects are completely different in both systems, so it is hard to do a clean comparison of the absolute values and the trends should be compared instead. After close examination, the trends do not always appear to be the same for the two reaction systems. For example, CuO, HMF (deposited in CF) in batch had the worst performance in batch but CuO, HMF had the second-best performance in RDE. Similarly, CuO/NiO (deposited in CF) had the best performance in batch cell (with and without HMF) while CuO/Fe2O3, HMF had the best performance in RDE. Are the changes in trends associated with the deposition of the metal on the CF for batch (and flow) testing or is it experimental error? The authors must address this difference in trend.
We thank the reviewer for the comment. We double-checked our experimental data and can confirm that there was no experimental error. First, in Supplementary Fig. 17, we show both the influence of the different cells (RDE and UTEC) and the CF on the activity and mentioned this in the Main manuscript (page 9).
At the same time, however, Supplementary Fig. 17 shows that CF alone does not necessarily lead to a generally increased performance. Here it becomes clear once again that the combined CuO/MOx metal oxides with or without CF support bring an increase in activity. Supplementary Fig. 17 However, we would like to reiterate that there are differences in mass transport between the two cell configurations. In the RDE setup, we apply rotation while the electrode is static in the UTEC setup. By the rotation in the RDE, the HMF transport to the electrode and the products' removal is likely accelerated. In addition, it can be assumed that the transport of the produced hydrogen from the electrode to the solution is also accelerated, reducing the blockage of active sites at the catalyst. These reasons can therefore lead to a change in activity in addition to the CF.
In our new data, CuO/NiO with HMF (RDE) and CuO/NiO/CF with HMF (UTEC) show comparable performances up to 10 mA cm -2 ( Supplementary Fig. 17a). At higher current densities, we see that the CuO/NiO/CF with HMF (UTEC) shows higher performance compared to the CuO/NiO catalyst (RDE). But it must be noted that in the UTEC configuration, there is no difference between the curves with and without HMF. Here the HER also starts after 0.2 VRHE. In the RDE configuration, the exponential drop of the curve, which we associate with an increasing HER, is not visible before 0.5 VRHE. This difference in onset potentials is caused by the previously mentioned mass transport limitations in the UTEC.
In addition, the CF and, thus, the associated expansion of the three-dimensional surface area and the CF's porosity influence the activity. We also assume mass transport limitations as the reason for the decrease in performance from RDE to UTEC for the CuO/CF catalyst with HMF. However, compared to the CuO/NiO catalyst, CuO shows a bad HER activity which is why, for example, the increase in surface area by CF for the CuO/CF catalyst in UTEC does not show the same increase in activity as for the CuO/NiO/CF catalyst.
2. Additionally the shape of the curves shown in Figure 3c of some CuO/MOx systems (CuO,HMF, CuO/NiO,HMF and CuO/Co3O4,HMF as well as CuO/Fe2O3 to some extend) in the RDE do not follow the traditional exponential trend. Hence, the authors are encouraged to revise the data and perform different IR corrections to match the data between the RDE and batch cell.
We agree with the reviewer that the shape of the curves in Figure 3c changes for the catalysts mentioned and no longer follows the "classical" exponential shape we all know from the textbooks. We would even extend this observation to parts of Figure 3a and b. It is striking that this only occurs for the curves measured with HMF-containing electrolyte. This is understandable since, after adding HMF to the electrolyte, it is no longer a "classic" one-reaction process system. HMFRR and HER take place simultaneously, which leads to a changed shape of the curves.
We would like to abstain from modified IR correction, as we think our 100% manual IR correction is accurate.
Electrochemical performance of the AEM-flow system 1. The authors indicated in the rebuttal letter that they cannot place a reference electrode in the flow cell. Per Figure 3g, there is a 0.3 V difference between the CuO/MOx electrodes to generate the same current, so the potential at which the electrodes operated in the flow-cell to generate the same current is also different. How do the authors know that the small difference in performance observed with different secondary metal at 10 mA/cm 2 is caused by the nature of the secondary metal and not by operating at each system at different potentials? The authors must address this critical point. Since the half-cell potential cannot be measured, the authors are recommended to operate the flow system at different currents (i.e., potentials) but similar conversion levels to demonstrate whether the current/potential affect the product selectivity for a given CuO/MOx combination.
We agree that different catalysts lead to different cell potentials at similar constant current densities. We have chosen a galvanostatic reaction technique and applied constant current density for 30 min reaction time to ensure that the same amount of charge is transferred to each electrode. With a potentiostatic technique, we keep the cell potential constant, but the current and, thus, the amount of charge varies. We would have a similar problem if we measured up to different conversion levels. Each catalyst would need a different reaction time to reach a given conversion at constant current density, and accordingly, a different amount of charge would be transferred.
However, in the following table (Table R1), we have summarized the performance parameters for all three current densities to show that the conversions and the cell potentials (after 30 min) are in a comparable range for the different electrodes (catalysts) at the given current densities. It should be noted that a comparison of the cell potentials is only permissible under the assumption that the other components of the cell, such as the counter electrode or membrane, always make the same contribution to the cell potential. We would like to emphasize once again that the cell potentials, as well as the conversions, differ from each other within a reasonable range. Thus the statement about the change in activity due to the second metal is and remained valid in our view. Table S3 summarizes the performance of the flow cell after 30 min of reaction. All the catalytic systems were operating at ≈90% HMF conversion at 20 mA/cm 2 ; hence, the discussion of changes in selectivity with current is not appropriate because the main reagent is mostly consumed, and the system is "forced" to do side reactions (H2) to produce a constant rate (current). The authors should operate the system at a similar, lower conversion level (≈10%) for all the test to truly observe the effects of current. The authors are encouraged to run the system with 10X higher concentration of HMF, lower catalyst loadings, and/or lower current densities as well.

Figures 4a to 4e and
We thank the reviewer for this reasonable comment and agree that with decreasing HMF concentration, the FEHER increases (Figure 4f). We also agree that a constant high HMF concentration (XHMF≤10%) must be provided over the entire reaction time for more fundamental investigations. In order to measure constantly at such a conversion in our cell without changing the reaction conditions, our reaction time would be greatly reduced to less than 5 min, which at a flow of 25 ml min -1 could mean that the reaction would end before the electrolyte (V=100 ml) has completely flowed through the cell once. However, a 10x higher HMF concentration is not an option since at high HMF concentrations in alkaline electrolytes, parasitic side reactions to humins occur more frequently. 1,2 Thus, our initial HMF concentration would steadily decrease and quickly fall below the 10% conversion limit.
Based on our loading study ( Supplementary Fig. 16), lowering the catalyst loading at 10 mA cm -2 isn't an option either because it would lead to increased cathodic half-cell potentials and, thus, to a more dominant HER.
Lowering the current density to, for example, 5 mA cm -2 would indeed counteract a too-rapid HMF conversion, a half-cell potential increase, and stronger HER. Still, measuring at such low current densities is rather unusual in a 5 cm 2 electrode flow cell.
In summary, we understand the importance of the reviewer's concerns but find them currently very difficult to impossible to implement in our study for the reasons mentioned above. Especially we also think that the flow cell configuration is not the most suitable here. At the same time, we keep these important points in mind for another study with a different focus.
3. Figure 4c shows the selectivity of CuO/NiO/CF at 10, 20, 30 mA/cm 2 after 30 min of reaction and Figure 4f shows the selectivity of the CuO/NiO/CF at 20 mA/cm 2 at 0-15, 15-30, 30-45, and 45-60 min. This reviewer would expect the selectivity of CuO/NiO/CF in Figure 4c at 20 mA/cm 2 and 30 min to be the same as the selectivity in Figure 4f at 30 min and 20 mA/cm 2 but it is not. Figure 4c shows a 20% H2 and 27% MFA selectivity while Figure 4f shows 71% H2 and 0% MFA selectivity. Why are the results so different even when the electrode, current, and reaction time are the same? The authors must address this inconsistency.
We thank the reviewer for the comment. The difference between these two plots is that in Figure 4c, the entire period of 30 minutes is considered, and in Figure 4f, 15-minute time intervals are considered. For a better understanding, we added more information to the caption of Figure 4f (main manuscript, page 12).
f) Faradaic efficiencies and HMF conversion (yellow) are calculated for every 15 min time interval over 60 min using CuO/NiO/CF as a catalyst.